Vacuum imaging drum with a material receiving recess in the periphery thereof

ABSTRACT

An imaging system comprising a source of light movable with respect to a writing element and projectable thereon to generate an image, a focusing means is provided for focusing a light source which generates a first beam of light of a wavelength selected to be actinic with respect to the writing element. At least a portion of the first beam of light is absorbed by the writing element. The apparatus comprises means to automatically supply donor sheets and receiver sheets independently to a writing platen or drum, and to selectively load and unload the donor sheets from superposition with the receiver sheet without disturbing the registration of the receiver sheet. Also included are components which contribute to the quality of the images generated, such as relatively inexpensive, low jitter transports and drives for the writing head and imaging drum. In particular, the writing drum is provided with an axial flat and a circumferential recess to facilitate loading, holding and unloading the donor and receiver sheets and to assure a smooth surface onto which the image is formed.

RELATED APPLICATIONS

The present application is related to the following commonly assignedco-pending applications: U.S. Ser. No. 670,088, now U.S. Pat. No.5,196,242 entitled WRITING BEAM ANGULAR ALIGNMENT DEVICE; U.S. Ser. No.670,089, now U.S. Pat. No. 5,013,121 entitled AUTOMATIC CUT-OUT FORAUTO-FOCUS DEVICE; U.S. Ser. No. 670,092, entitled WRITING BEAM FOCUSINGUTILIZING LIGHT OF A DIFFERENT WAVELENGTH; U.S. Ser. No. 670,095, nowU.S. Pat. No. 5,196,866 entitled FOCUS FIBER MOUNT; and U.S. Ser. No.670,129, now U.S. Pat. No. 5,138,497 entitled HIGH SPEED FOCUSING LENSASSEMBLY, all filed on Mar. 15, 1991; U.S. Ser. No. 749,229, entitledLASER THERMAL PRINTER METHOD AND APPARATUS, in the names of Raymond J.Harshbarger William G. Fahey and Ronald R. Firth; U.S. Ser. No. 749,378,entitled SELECTIVELY WOUND MATERIAL FOR A LASER THERMAL PRINTER, in thename of Michael H. Parsons; U.S. Ser. No. 749,223, entitled MATERIALSUPPLY CAROUSEL, in the names of James L,. Mohnkern, Michael H. Parsonsand Rene L. Gobeyn; U.S. Ser. No. 749,050, entitled MATERIAL TRANSPORTUTILIZING A MOVABLE EDGE GUIDE, in the name of Michael H. Parsons; U.S.Ser. No. 749,372, entitled LASER THERMAL PRINTER WITH A VERTICALMATERIAL TRANSPORT, in the name of Michael H. Parsons; U.S. Ser. No.749,224, entitled MATERIAL TRANSPORT THAT SELECTIVELY CONTACTS DIFFERENTMATERIALS, in the names of Michael H. Parsons and William J. Simmons;U.S. Ser. No. 749,399, entitled MULTI-CHAMBERED IMAGING DRUM, in thename of Roger S. Kerr; U.S. Ser. No. 749,232, entitled METHOD ANDAPPARATUS FOR SELECTIVELY SORTING IMAGE-BEARING SHEETS FROM SCRAPSHEETS, in the names of Bradley C. DeCook, Roger S. Kerr and Richard L.O'Toole; U.S. Ser. No. 749,231, entitled METHOD OF REMOVING AIR FROMBETWEEN SUPERPOSED SHEETS, in the names of Bradley C. DeCook, Roger S.Kerr and Richard L. O'Toole; U.S. Ser. No. 749,389, entitled VACUUMIMAGING DRUM WITH AN AXIAL FLAT IN THE PERIPHERY THEREOF, in the name ofRoger S. Kerr; U.S. Ser. No. 749,230, entitled METHOD AND APPARATUS FORLOADING AND UNLOADING SUPERPOSED SHEETS ON A VACUUM DRUM, in the namesof Roger S. Kerr and James K. Lucey; U.S. Ser. No. 749,227, entitledLASER THERMAL PRINTER WITH POSITIVE AIR FLIOW, in the names of Roger S.Kerr and Douglass L. Blanding; U.S. Ser. No. 749,226, entitledAUTO-FOCUS DETECTOR MASK, in the name of Michael S. Ferschl; U.S. Ser.No. 749,225, entitled INITIAL SET-UP PROCEDURE FOR AN AUTO-FOCUS LENS,in the name of Michael S. Ferschl; U.S. Ser. No. 749,222, entitledFOCUSING LASER DIODE MOUNT ON A WRITE HEAD, in the names of Michael S.Ferschl and Erich Zielinski; U.S. Ser. No. 749,386, entitled OPTICALFIBER MOUNT AND SUPPORT, in the names of Roger S. Kerr and Stanley J.Thomas; U.S. Ser. No. 749,387, entitled REGISTRATION INDICIA ON A DRUMPERIPHERY, in the names of Cheryl J. Kuberka, David F. Dalfonso andEnsley E. Townsend; U.S. Ser. No. 749,382 PRECISION LEAD SCREW DRIVEASSEMBLY, in the name of Erich Zielinski; U.S. Ser. No. 749,390,entitled OPTICAL FIBER TAKE-UP ASSEMBLY, in the name of Erich Zielinski;U.S. Ser. No. 749,383 WRITING TRANSLATOR MOUNT, in the name of ErichZielinski; U.S. Ser. No. 749,396, entitled HIGH APERTURE FINITECONJUGATE LENS SYSTEM SUITABLE FOR USE AS A MICRO RELAY LENS, in thename of Donald DeJager, all filed simultaneously herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color proofing apparatus whichutilizes an electronic signal input, and more particularly, to a methodand apparatus for automatically producing full-color proof images usinglasers to provide thermal energy to a series of color dye-donors toselectively transfer each dye in registration to a receiver to form aproof image.

2. Description of the Prior Art

Color-proofing is the procedure used by the printing industry forcreating representative images that replicate the appearance of printedimages without the cost and time required to actually set up ahigh-speed, high-volume printing press to print an example of the imagesintended. In the past, these representative images, or proofs, have beengenerated from the same color-separations used to produce the individualcolor printing plates used in printing presses so that variations in theresulting images can be minimized. Various color-proofing systems havebeen devised to create the proofs and have included the use of smaller,slower presses as well as means other than presses, such asphotographic, electrophotographic, and non-photographic processes.

The proofs generated are judged for composition, screening, resolution,color, editing, and other visual content. The closer the proofreplicates the final image produced on the printing press, as well asthe consistency from image to image, from press to press, and from shopto shop, the better the acceptance of the proofing system by theprinting industry. Other considerations used in judging proofing systemsinclude reproducibility, cost of the system as well as cost of theindividual proofs, speed, and freedom from environmental problems.Further, since nearly all printing presses utilize the half-tone processfor forming pictorial images, wherein the original image is screened,i.e. photographed through a screen to produce one or more printingplates containing an image formed of a plurality of fine dots thatsimulate the varying density of the original image, proofing processesthat employ the half-tone process to form the proof image have beenbetter accepted by the printing industry than have continuous tonesystems.

With the advent of electronic systems for the generation of printingplates from electronic data stored in appropriate data storage devicesin the form of electronically separated single color separations, theuse of photographic color separations for generating proof images hasbecome somewhat archaic, and a variety of processes have been developedand implemented to electronically form, store, and manipulate imagesboth for generating the actual printing plates as well as for generatingthe proof images. While some of these electronic systems can handle andproduce analog images, the most widely used systems employ digitalprocesses because of the ease of manipulation of such digital images. Ineach of these electronic processes it is possible to display theresulting image on a CRT display, but it is generally necessary toproduce a "hard copy" (i.e. an image actually formed on a sheet of paperor other material) before it can be fully assessed for approval of thefinal printing operation. Thus, each of these electronic systemsrequires the use of some form of output device or printer which canproduce a hard copy of the image for actual evaluation. It is to thefield of proofing output devices that the present invention is directed.

While purely photographic processes can provide accurate reproductionsof images, they do not always replicate the reproduction resulting fromprinting presses. Further, most photographic processes do not producehalf-tone images that can be directly compared to the printed imagesthey are intended to emulate. Moreover, they are almost universallyincapable of reproducing the images on the wide variety of paper orother material that can be run through a printing press. It is knownthat the appearance of the final printed image is affected by thecharacteristics of the paper or other material upon which it is printed.Thus, the ability to form the proof image on the material actually to beused in the press can be a determining factor in the market success ofthe proofing system.

Other continuous tone proofing systems, such as thermal processes andink-jet systems have been developed, but they do not replicate thehalf-tone images so desired by the printing industry.

Electrophotographic proofing systems with half-tone capability have beenintroduced which employ either wet or dry processes. Theelectrophotographic systems that use dry processes suffer from the lackof high resolution necessary for better quality proofing, particularlywhen the images are of almost continuous tone quality. This results fromthe fact that dry electrophotographic processes do not employ tonerparticles which have a sufficiently small size to provide the requisitehigh image resolution. While wet electrophotographic processes do employtoners with the requisite small particle size, they have otherdisadvantages such as the use of solvents that are environmentallyundesirable.

In commonly assigned U.S. patent application Ser. Nos. 451,655 and451,656, both filed Dec. 18, 1989, a thermal printer is disclosed whichmay be adapted for use as a direct digital color proofer with half-tonecapabilities. This printer is arranged to form an image on a thermalprint medium, or writing element, in which a donor element transfers adye to a receiver element upon receipt of a sufficient amount of thermalenergy. This printer includes a plurality of diode lasers which can beindividually modulated to supply energy to selected areas of the mediumin accordance with an information signal. The print-head of the printerincludes one end of a fiber optic array having a plurality of opticalfibers coupled to the diode lasers. The thermal print medium issupported on a rotatable drum, and the print-head with the fiber opticarray is movable relative to the drum. The dye is transferred thereceiver element as the radiation, transferred from the diode lasers tothe donor element by the optical fibers, is converted to thermal energyin the donor element.

A direct digital color proofer utilizing a thermal printer such as thatjust described must be capable of consistently and accurately writingminipixels at a rate of 1800 dots per inch (dpi) and higher to generatehalf-tone proofs having a resolution of 150 lines per inch and above, asis necessary to adequately proof high quality graphic arts images suchas those found in high quality magazines and advertisements. Moreover,it is necessary to hold each dot or minipixel to a density tolerance ofbetter than 0.1 density unit from that prescribed in order to avoidvisible differences between the original and the proof. This densitycontrol must be repeatable from image-to-image and frommachine-to-machine. Moreover, this density control must also bemaintained in each of the colors being employed in multiple passesthrough the proofer to generate a full color image.

Aspects of the apparatus which affect the density of the dots that makeup the image include such things as variations and randomness of theintensity and frequency of the laser output, and variations in theoutput of the fiber optics which can vary from fiber to fiber and evenwithin a single fiber as it is moved during the writing process.Variations in the finish of the drum surface as well as drum runout anddrum bearing runout and variations in the parallelism of the translationof the print-head with respect to the axis of the drum also affect thedensity of the image dots. Any uneven movement of the imaging drum or ofthe writehead translation during the writing process, or anything whichimparts jitter to any part of the imaging apparatus can adversely impactthe quality of the finished image and its value as a representativeproof. The difference in the distance between the ends of individualfibers and the drum surface also affects image density because of thefact that the end of the fiber bundle is flat while the surface of thedrum is curved. Temperature variations in the print-head due to theambient temperature of the machine as well as the fact that the writingprocess itself heats the print-head also influence the image density.

Variations in the print medium elements, such as variations in thethickness of the donor and receiver elements as well as the variouslayers that are a part thereof, can also affect the image density as itis being written.

Thus, it has been found necessary to provide a writing apparatus whichmeets all of the foregoing requirements and yet which is substantiallyautomated to improve the control, quality and productivity of theproofing process while minimizing the attendance and labor necessary.Moreover, the writing apparatus must be capable of not only generatingthis high quality image consistantly, but it must be capable of creatinga multi-color image which is in registration regardless of how thevarious individual images are supplied to the element comprising thefinal image. That means, in a thermal proofing process such as thatdescribed above, that various donor material sheets must be sequentiallysuperposed with a single receiver sheet and then removed withoutdisturbing the receiver sheet on the writing drum or platen, sincemaintaining the receiver sheet in one position during the entire writingprocess is what assures the necessary registration of the multiplesuperposed images that create the final proof.

Thus it will be seen that a method and apparatus for constantly, quicklyand accurately generating an image utilizing such a thermal imagingprocess to create high quality, accurate, and consistant proof imageswould be technologically desirable and economically advantageous.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, in a thermalimaging apparatus is provided which comprises a support member arrangedto mount a receiver member and a donor member in superposed relationshipthereon. Means is provided for generating a modulated coherent lightbeam and for projecting the light beam onto the donor member mounted onthe support member to transfer an image onto the receiver member bytransferring a dye from the donor member. Means is provided for movingthe light beam relative to the donor member on the support member.Supply means is provided for selectively supplying a sheet of a receivermaterial and a sheet of a donor material to the support member. Thesupport member is provided with sheet holding means in the surfacethereof for adhering a receiver sheet and a donor sheet to the supportmember in superposed relation. First means is provided for selectivelyremoving the donor sheet from superposition with the receiver sheet onthe support member, and second means is provided for selectivelyremoving the receiver sheet containing an image from the support member.

According to another embodiment of the present invention a thermalimaging apparatus comprises an imaging drum member mounted for rotationabout its axis and arranged to mount a receiver member and a donormember in superposed relationship thereon. Means is provided forgenerating a modulated coherent light beam and for projecting the lightbeam onto the donor member mounted on the drum member to transfer animage onto the receiver member by the transferance of a dye from thedonor member. Means is provided for moving the light beam projectionmeans axially along the drum member and for coordinating the motion ofthe drum member with the motion of the light projection means torepeatably register the image on the receiver member. A supply meansselectively supplies a sheet of a receiver material and a sheet of adonor material to the support member. The drum member is provided withvacuum sheet holding means for adhering a receiver sheet and a donorsheet to the drum member in superposed relation. First means is providedfor selectively removing the donor sheet from superposition with thereceiver sheet on the drum member, and second means is provided forselectively removing the receiver sheet containing an image from thedrum member.

In a further embodiment of the present invention a thermal imagingapparatus comprises a horizontal imaging drum member mounted forrotation about its axis and arranged to mount a receiver member and adonor member in superposed relationship thereon. Means is provided forgenerating a plurality of modulated coherent light beams and forprojecting the light beams as a line inclined at an angle to the axis ofthe drum member onto the donor member mounted on the drum member totransfer an image onto the receiver member by transferring a dye fromthe donor member. Means is provided for reversibly rotating the drummember and means is provided for moving the projecting means axially ofthe drum member. A supply means is arranged to selectively supply areceiver member and a plurality of donor members to the drum member,with the supply means being arranged to supply a plurality of donorsheets sequentially to be individually superposed and imaged on a singlereceiver sheet. The receiver member has a first width and the donormembers have a second width greater than that of the receiver member.Means is provided for selectively registering the different sheetsaxially along the length of the imaging drum member. A loading roller isarranged to selectively engage the surface of the imaging drum member toengage the donor sheets and to smoothe them into engagement with thesurface of the receiver sheet on the imaging drum member. A first meansis operable to engage an end edge of a donor sheet to remove it fromsuperposition with the receiver sheet when the imaging drum member isrotated in a first direction without moving the receiver sheet, andsecond means is operable to engage an end edge of a receiver sheet toremove it from the drum member when the drum is rotated in a seconddirection.

In a still further embodiment of the present invention, a method ofthermal imaging is provided comprising the steps of selectivelysupplying a receiver member and a plurality of donor members to ahorizontal imaging drum member mounted for rotation about its axis. Theplurality of donor sheets are supplied sequentially and are individuallysuperposed and imaged on a single receiver sheet. A receiver member anda donor member are mounted in superposed relationship on the drum memberand are selectively registered axially along the length of the imagingdrum member. A loading roller is selectively engaged with the surface ofthe imaging drum member to engage the donor sheets and to smoothe theminto engagement with the surface of the receiver sheet on the imagingdrum member. The drum member is rotated and a plurality of modulatedcoherent light beams are generated and projected as a line inclined atan angle to the axis of the drum member onto the donor member totransfer an image onto the receiver member by the transference of a dyefrom the donor member. The projecting means is moved axially of therotating drum member, The imaging drum member is rotated in a firstdirection and an end edge of a donor sheet is engaging to remove it fromsuperposition with the receiver sheet without moving the receiver sheet.The drum member is rotated in a second direction and an end edge of areceiver sheet is engaged to remove the receiver sheet from the imagingdrum.

Various means for practicing the invention and other features andadvantages thereof will be apparent from the following detaileddescription of illustrative, preferred embodiments of the invention,reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation schematic view of a proofing printer of thepresent invention;

FIG. 2 is a front perspective view of a material supply carousel;

FIG. 3 is a rear perspective view of the material supply carousel;

FIG. 4 is an enlarged perspective view of a material feed drive;

FIG. 5 is a perspective view of the sheet transport assembly;

FIG. 5a is a cross sectional view taken along line 5a--5a FIG. 5;

FIG. 6 is a perspective view of the imaging drum and the write head ofthe present invention, partially cut-away to reveal hidden portionsthereof;

FIG. 7 is a cross sectional view of the writing head and lens assemblytaken along line 7--7 of FIG. 6;

FIG. 8 is a schematic elevation of the write head translation drive;

FIG. 9 is a schematic side view of the write head translation drivetaken along line 9--9 of FIG. 8;

FIG. 10 is a rear perspective view of the imaging drum and the writehead;

FIG. 11 is a partial rear perspective view of the imaging drum and thewrite head taken from the opposite end of the drum shown in FIG. 10;

FIG. 12 is a cross section of the imaging drum and its drive assembly;

FIG. 13 is a cross sectional view of the imaging drum taken along line13--13 of FIG. 12 illustrating the location of the vacuum chambers;

FIG. 14 is a cross sectional view of the receiver unload vacuum chambertaken along line 14--14 of FIG. 13 showing the details of the vacuumcontrol valve;

FIG. 15 is a view of the generated surface of the imaging drum;

FIG. 16 is an elevation view of the vacuum control valve actuator camtaken along line 16--16 of FIG. 17;

FIG. 17 is a side view of the actuator cam showing the actuation of thedonor load and unload vacuum control valve by the actuator cam;

FIG. 18 is an illustration similar to FIG. 17 with the actuator camdisengaged from the vacuum control valve;

FIGS. 19a-19c are elevation views of the actuator cam taken along line19--19 of FIG. 17 illustrating the relative positions of the camfollowers of the respective vacuum control valves for differentfunctions;

FIG. 20 is a schematic end view of the imaging drum illustrating thevarious operating positions thereof;

FIGS. 21a-21c are schematic illustrations of the application of vacuumto various regions of the imaging drum;

FIGS. 22a-22i are partial schematic illustrations of the materialhandling system at selected steps during the loading and unloading ofmaterial to and from the imaging drum;

FIG. 23, including FIGS. 23A-23C, is a tabulation of drum operatingconditions at each of the steps of material loading and unloading;

FIG. 24 is a perspective view of a laser mount and fiber guide;

FIG. 25 is an exploded view of the laser mount and fiber guide shown inFIG. 24;

FIG. 26 is a schematic view of the air flow through the proofing printerof the present invention;

FIG. 27 is a cross section through the optical fiber spool for thefocusing laser assembly mounted on the writehead; and

FIG. 28 is a plan view of the focusing photodetector taken along line28--28 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT Overall Apparatus

The overall laser thermal printer proofer 10 of the present invention isillustrated in FIG. 1 and comprises generally a material supply assembly12, a sheet cutter assembly 14, a sheet transport assembly 16, animaging drum 18, a writehead assembly 20, a waste transport 22, and animage exit transport 24, which will all be described in greater detailhereinbelow. The arrangement of the components within the enclosure orcabinet 26 is such that the imaging drum 18 and the writehead assembly20 are disposed in the upper central region of the cabinet. The materialsupply assembly 12 is disposed in the lower portion at one end of thecabinet, with the sheet cutter assembly 14 disposed adjacent thematerial supply assembly, again in the lower portion of the cabinet. Thesheet transport assembly 16 extends from the sheet cutter assembly 14 toadjacent the imaging drum 18, generally opposite to the writeheadassembly 20.

The overall operation of the apparatus comprises removing a portion ofthe supply of a receiver material from the material supply assembly 12,measuring it and cutting it to length in the sheet cutter assembly 14,and then transporting the cut sheet via sheet transport assembly 16 tothe imaging drum 18 about which it is wrapped, registered, and secured.A length of donor material is then removed from the material supplyassembly 12, cut to length by the sheet cutter assembly 14 andtransported by the sheet transport assembly 16 to the imaging drum 18.At the imaging drum the donor material is wrapped around the drum andsuperposed in the desired registration with the receiver materialalready secured thereon. After the donor material is secured to theperiphery of the drum, the writehead assembly is traversed axially alongthe drum as the drum is rotated, and an image is imparted to thereceiver sheet. After the image has been written on the receiver sheet,the donor sheet is removed from the imaging drum, without disturbing thereceiver sheet, and transported out of the apparatus via waste exittransport 22. Additional donor sheets are sequentially superposed withthe receiver sheet on the drum and are imaged onto the receiver untilthe desired image is obtained and the completed image is exited from theapparatus via the image exit transport 24. A more detailed descriptionof the apparatus and the operation thereof follows.

Heretofore, thermal imaging apparatus has employed one of two materialhandling approaches; either all of the material has been supplied andfed in sheet form, or one element has been supplied and fed as a sheetand the other has been supplied and fed as a web. Each of theseapproaches has certain disadvantages. The supply of material in the formof sheets increases the cost of manufacturing the material as well asincreases the probability of double feeding of the sheets. Since anydouble feeding of any of the sheets would render that imageunacceptable, as well as risk damage to the apparatus, this is a lessdesirable approach. The supply and use of one of the materials in webform necessitates the use of an image writing assembly that is capableof writing an entire line at one time and thus a very long writing arrayto produce the entire line. Such long writing arrays are difficult andcostly to manufacture, but are even more difficult to accurately controlover the entire line to provide the image consistency required for proofquality images. Accordingly, the present invention provides the abilityto produce proof quality images from material which is supplied and fedin web form to reduce both the cost and the problem of double feeding,and yet cuts the web into sheet form before use so that the sheets maybe accurately superposed onto a rotating imaging drum that may bewritten on by a writing array that is relatively small and provides aconsistent image across the entire image, as well as being relativelyeasy and inexpensive to manufacture. Further, with the supply of thematerial in roll form, with automatic feeding and cutting thereof, themachine cabinet can be sealed against the entry of environmental dirtthat could otherwise enter the unit and possibly adversely effect imagequality. Thus, the present invention provides the advantageousattributes of both kinds of thermal imaging apparatus of the prior art.

Material Supply Assembly

The material supply assembly 12 is illustrated in FIGS. 1-4 andcomprises a carousel assembly 30 mounted for rotation about a horizontalaxis 31 on bearings 32 and 34 at the upper ends of vertical supports 36and 38. The carousel comprises a vertical circular plate 40 having aplurality of material supporting spindles 42 cantilevered outwardly fromand equispaced about the front face of the plate. Each of the spindles42 is arranged to carry a roll supply of material 44 for use on theimaging drum 18. In the embodiment illustrated in FIGS. 1, 3, and 26,seven spindles are provided, with one spindle arranged to carry a rollof receiver material, and four spindles arranged to carry four differentrolls of donor material to provide the four primary colors used in thewriting process, e.g. cyan, magenta, yellow, and black. Two additionalspindles are provided to give the operator the opportunity to alsoinstall specialty colors, if desired, without the need of removing anyof the other donor or receiver rolls. Alternatively, a greater or lessernumber of spindles may be provided depending upon the needs of the user.For example, FIG. 2 illustrates a carousel having six spindles. Each ofthe material supply spindles is provided with a corresponding materialfeeder assembly 46, only one of which is illustrated in FIG. 1. Each ofthe material feeder assemblies is arranged to withdraw the end of theroll material from the rolls 44 carried on the spindles 42. The end ofthe material is fed between the nip of a driven roll 48 and a pair ofpressure rollers 50 and is fed out of the carousel through guides 52 andcorrugating rollers 54. The drive roller 48 is mounted on a shaft 56which extends through the carousel plate 40 to the rear side thereof andwhich is provided with a driven gear 58, to be described more thoroughlyhereinbelow. The pressure rollers 50 are mounted on an assembly 60 whichis provided with a spring 62 which urges the pressure rollers intoengagement with the drive roller 48. The edge guides 52 are arranged tolocate the end of the web of material to prevent it from drooping duringperiods when the equipment is not in operation. The corrugating rollers54 are arranged to impart a slight corrugation to the material acrossthe width thereof to provide increased structural rigidity thereto tofacilitate guiding it into the sheet cutter assembly.

The roll material is provided to the apparatus an flangeless cores toeconomize cost and weight. The flanges for the rolls are part of thespindles of the carousel. The weight of the roll of material issufficient to keep the roll from telescoping, clockspringing, orunwinding unless the material is driven by the drive roller 48.

The carousel 30 is rotated about its axis to bring a selected rollsupply of material into opposition with the sheet cutter assembly 14where the material is removed from the roll supply, is fed through thecutter assembly 14, is measured, and is then cut. The carousel isrotated, counterclockwise in FIG. 1, by means of a drive motor 64driving a sheave 66 which engages a belt 68 that is tensioned about theperiphery of the carousel plate 40. A brake assembly 70 is arranged tohold the carousel stationary when it is not being driven by motor 64. Acommon drive assembly 72 is provided for all of the material feederassemblies 46, and is also illustrated in FIG. 3. The drive assemblycomprises a drive gear 74 disposed at the end of a rotatable arm 76. Thegear 74 is driven by motor 78 and the arm 76 is rotated by motor 80between an at rest position, shown, wherein the drive gear 74 is locatedout of the path of the driven gears 58 associated with each materialsupply assembly as the carousel is rotated, and an operative positionwherein the drive gear 74 engages the driven gear 58 of the materialfeed assembly located at the material feed location to provide power tothe drive roll 48 to feed the material from the roll supply to the sheetcutter assembly. Also illustrated in FIG. 3 is an exhaust fan assembly82 which operates to exhaust air from the sheet cutter assembly 14, aswill be described more thoroughly hereinbelow.

In the preferred embodiment, the roll supplies of donor and receivermaterials provided to the printer apparatus are preferentially eachwound on their respective cores in a unique fashion. The receivermaterial, on to which the image is to be transferred by the write head,and which is the surface of that web which is most sensitive to contactdamage, is wound with the receiving surface on the outer surface of theweb as it is wound upon the core. On the other hand, the donor material,with an active surface from which the dye material is transferred ontothe receiver material, and which surface is most sensitive to contactdamage, is wound with the dye material on the inner surface of the web.As a result, these two material may be supplied over a common path,using common transport apparatus without the "sensitive" surfaces ofeach material being adversely contaminated by contact with transportsurfaces which have previously contacted and possibly been contaminatedby the "active" surface of the other material. In addition, each of thematerials is laid onto the imaging drum with the proper surfaceorientation, i.e. with the active surfaces facing each other. Thus, thereceiver sheet is transported with the support of the material down andthe receiver surface up, and the donor material is transported with thesupport surface up and the active donor surface down, without damagingeither of the respective "sensitive" surfaces. In addition, it will beseen that any curl in the respective materials, from being wound in thesupply roll, will match and compliment the curvature of the imaging drumwhen the material is supported thereon, simplifying the feeding andadherence of the respective materials to the imaging drum.

Sheet Cutter Assembly

The sheet cutter assembly 14 (FIG. 1) is disposed adjacent the materialsupply carousel 30 at the material feed location and is arranged toreceive the end of the web material as it is fed by the material feedassembly 46. The sheet cutter assembly comprises a mating pair of cutterblades 84 through which the web material is moved by the material feedassembly 46. A material metering drum 86 and mating endless drive belt88 cooperate to engage the web material as it is driven between thecutter blades 84, to assist the feeding thereof, and to change its pathfrom substantially horizontal to a generally vertical direction. Themetering drum and a sensor, not shown, are arranged to sense the end ofthe web material being fed and to determine when the desired length ofthe sheet has been fed between the cutter blades 84. At that point, themetering drum 86 and the cooperating belt 88, as well as the driveassembly 72, are stopped and the cutter blades are actuated to chop asheet member from the end of the web material. The web meteringarrangement is capable of providing sheets having two different lengths,as is necessary when cutting the receiver material and the donormaterial, since it is desired to form the donor material with a greaterlength than the receiver material so that it overlies the leading andtrailing ends of the receiver material when they are superimposed uponthe imaging drum. The material metering drum 86 and its cooperatingendless belt 88 are so constructed that they gently engage the materialbeing transported so that there is no relative motion between thematerial and the surfaces of the drum and the belt. Thus, the meteringdrum and belt do not scratch or otherwise damage the sensitive surfaceof the material being fed.

Sheet Transport Assembly

After a sheet has been cut from the end of the roll of material, it isdischarged from the metering drum and belt, generally upwardly into avertical sheet transport assembly 16, which is shown in greater detailin the perspective view of FIG. 5. The sheet transport assemblycomprises an upwardly directed air table 90 having a plurality of airholes, not shown, opening therethrough into an air chamber 92therebeneath. The air chamber is provided with a source of pressurizedair from a source to be described later, which air escapes through theholes in table 90 to support a sheet member thereon. The supply of airto the air chamber is controlled by a damper valve 93 (see FIG. 26) inthe inlet to the air chamber. A plurality of wire guides 94 aresuspended above the surface of table 90 to limit the upward movement ofthe sheet member due to the air flow through the air table. One edge ofthe air table is provided with an edge guide 96 which depends from aplate 98 which lies in substantially the same plane as the wire guides94. The edge guide is movable between a first position, shown in solidlines, and a second position, shown in dotted lines, by a cam mechanism100. Three pair of soft flexible drive rollers 102 and 104 are disposedalong the lateral edge of the air table adjacent the edge guide. Therollers 104 are conical in shape with the larger diameter disposedadjacent the edge guide 96. The rollers 104 form a nip with rollers 102and are arranged to gently engage the lateral edges of the sheets beingtransported thereby to drive the sheet toward the imaging drum and tourge it against the edge guide to provide the desired lateralregistration of the sheet with respect to the drum axis. The rollers 104are mounted on shafts 106 which are driven through gear boxes 108 whichare supplied with power from a motor, not shown.

The edge guide 96 is movable between two positions in order to properlydeliver the receiver sheet and the donor sheet to the imaging drum atthe proper axial location. As noted above, the receiver sheet is firstlaid onto the imaging drum and then a donor sheet is superposed over thereceiver sheet. To ensure complete coverage of the receiver sheet by thedonor sheet, and to facilitate the application of a vacuum to theinterface between the superposed donor and receiver, the donor sheet isarranged to extend beyond all four edges of the receiver sheet. Thus,the donor sheet must be wider than the receiver sheet. Accordingly, whenthe edge guide 96 is in its first position, shown in solid lines inFIGS. 5 and 5a, the receiver sheet is properly axially positioned alongthe length of the drum. When the edge guide is disposed in the secondposition, indicated by the dotted line illustration in FIGS. 5 and 5a,the donor sheet is properly axially positioned along the length of theimaging drum. As noted above, the tapered rolls 104 exert a lateralforce on the sheet engaged thereby gently forcing it into the edge guide96 to assure the proper lateral positioning of the receiver and thedonor sheets.

The operation of the material supply assembly comprises rotating thecarousel wheel 40 (FIGS. 1 and 4) so that the supply roll 44 of thereceiver material is located adjacent the sheet cutter assembly 14. Thearm pivot motor 80 of the feed assembly drive is operated to rotate thedrive gear 74 into engagement with the driven gear 58 of the respectivedrive roll 48. The gear drive motor 78 is then energized to rotate thedrive roll 48 to feed the end of the web of receiver material into thesheet cutter assembly 14. The metering roll 86 and the mating endlessbelt 88 are actuated, engaging the end of the receiver material web anddriving it along the path through the sheet cutter assembly until theend of the web is sensed and the appropriate length is drawn off thesupply roll 44. The metering roll 86 is then stopped and the cutterblades 84 are actuated to sever a sheet of receiver material from thereceiver supply roll. Inasmuch as the receiver material sheet is thefirst to be supplied to the imaging drum, the edge guide 96 of the sheettransport assembly is set in the first position, illustrated in solidlines in FIG. 5, to deliver the receiver sheet to the proper axiallocation on the imaging drum. Since the receiver material is driven withthe sensitive surface up as it is moved into the sheet transportassembly by the metering roll 86 and the belt 88, the air supply to theair table 90 is turned off by closing the damper valve 93 to the airchamber 92, so that the lower, support surface of the receiver materialsheet rests upon the air table 90. At this time, the rolls 102 and 104are actuated, moving the receiver sheet up the sheet transport assemblyto close proximity with the imaging drum 18, with the tapered rolls 104assuring that that edge of the receiver sheet is guided by the edgeguide 96. A sheet loading squeegee roller 97 is mounted axially of theimaging drum for selective engagement with the imaging drum at the upperend of the sheet transport assembly 16, as illustrated in FIGS. 1 and 5.The squeegee roller is so mounted that it can be moved by means of amotor or solenoid 99, see FIG. 5, into an operative loading position toforce a sheet being loaded onto the imaging drum into intimate contacttherewith to remove all air from the interface therebetween, or to anidle position out of the way of a sheet being fed to the drum. A sheetsensor 101 is disposed just above the squeegee roller 97 and is spacedfrom the surface of the drum sufficiently to be out of the paths of thesheet material being supplied to the drum (see FIG. 1). In the preferredembodiment, the sensor is responsive to light reflected from the surfaceof the drum or from a reflective surface of one of the sheets on orbeing supplied to the drum. The operation of the sensor 101 will be morethoroughly described with respect to the overall operation of theapparatus hereinbelow.

Since the receiver sheet is transported with the support surface downand the receiving surface up, the support surface of the sheet is theone which contacts the air table 90 and the sensitive receiver surfaceis not in contact with the guide wires 94. The drive rollers 104 onlycontact the receiver surface and are moving with the sheet and thus donot scuff or scrape the receiver surface.

After the receiver sheet has been attached to the imaging drum, as isdescribed hereinabove, the first donor material is supplied in a mannersimilar to that for the receiver material, except that the metering drumand the length sensor are arranged to generate a donor sheet which islonger than the receiver sheet. As the donor material is fed to thesheet transport assembly 16, the air table 90 is actuated by opening thedamper valve 93 to permit air flow therethrough and the donor sheet islifted by the air, away from the surface of the table into contact withthe guide wires 94. However, since the donor material is being fed withthe support surface up and the donor surface down, the relative motionof the donor support against the guide wires 94 causes no damage to thedonor surface. Similarly, the sensitive donor surface is contacted onlyby the rollers 102 which are driving the donor toward the imaging drum.Since the donor sheet is wider than the receiver sheet, and the donor isintended to overlap all of the edges of the receiver on the imagingdrum, the edge guide 96 is moved to the second position, shown in dottedlines, before the donor sheet is fed to the sheet transport assembly 16by the metering drum and belt 86 and 88. It will be seen that therespective "sensitive" surfaces of the receiver sheets and the donorsheets face different contacting surfaces in the sheet cutting assembly,the metering assembly, and the sheet transport assembly so that, shouldeither of the " sensitive" surfaces of either sheet be rubbed off ontoany of the apparatus surfaces, the "sensitive" surface of the othermaterial will not come into contact with the apparatus surfacescontaminated by the first material. In other words, the sensitivesurface of the receiver material only faces but does not normallycontact the guide wires 94, and since the guide wires 94 only contactthe support surface of the donor, the guide wires will not pick up anyresidue from the dye-containing surface of the donor. Similarly, sincethe air table 90 only contacts the support surface of the receivermaterial, it will not pick up any of the material from the receiver sidethereof, and since the donor material only "sees" but normally does notcontact the air table surface, the donor surface can not pick anyresidue up from the receiver sheet from the surface of the air table 90.

Imaging Assembly

Referring now to FIG. 6, a perspective view of the imaging drum 18 andthe writehead assembly 20 is illustrated. The imaging drum is mountedfor rotation about an axis 202 in frame member 204. The imaging drummember 18 is adapted to support a thermal print medium of a type inwhich a dye is transferred from a donor element to a receiver element asa result of heating the dye in the donor. The donor element and thereceiver element are superposed in intimate contact and are held ontothe peripheral surface of the drum member by means such as by vacuumapplied to the superposed elements from the drum interior as will bemore thoroughly described hereinbelow. A thermal print medium for usewith the printer proofer 10 of the present invention can be, forexample, the medium disclosed in U.S. Pat. No. 4,772,582, which includesa donor sheet having a material which strongly absorbs at the wavelengthof the exposing light source. When the donor element is irradiated, thisabsorbing material converts light energy to thermal energy and transfersthe heat to the dye in the immediate vicinity, thereby heating the dyeto its vaporization temperature for transfer to the receiver element.The absorbing material may be present in a layer beneath the dye, or itmay be admixed with the dye and is strongly absorptive to light havingwavelengths in the range of 800 nm-880 nm. An example of a preferredembodiment of a receiver element that can be used with the presentinvention is disclosed in co-pending, commonly assigned U.S. patentapplication Ser. No. 606,404, entitled Intermediate Receiver OpaqueSupport, and filed Oct. 31, 1990. The receiver element disclosed thereinincorporates a reflective layer which improves the efficiency of the dyetransfer to the receiver element.

An imaging light source is movable with respect to the imaging drum andis arranged to direct a beam of actinic light to the donor element.Preferably the light source comprises a plurality of laser diodes whichcan be individually modulated by electronic signals which arerepresentative of the shape and color of the original image, so thateach dye is heated to cause volatilization only in those areas in whichits presence is required on the receiver to reconstruct the color of theoriginal object. In the preferred embodiment, the laser diodes 206 aremounted remotely from the drum member 18, on the stationary portion ofthe frame 204, and each direct the light produced thereby to the inputend of a respective optical fiber 208 which extends to and transfers thelight to the movable writing head assembly 20 adjacent the drum member.The laser diodes are selected to produce a first beam of light havingwavelengths in the range of 800 nm-880 nm, and preferably predominatelyat a wavelength of 830 nm.

The writehead assembly 20 is moveably supported adjacent the imagingdrum 18 and comprises a writing head 218 which is mounted on a movingtranslator member 210 which, in turn, is supported for low frictionslidable movement on bars 212 and 214. The bars 212 and 214 aresufficiently rigid that they do not sag or distort between the mountingpoints at their ends and are arranged as exactly parallel with the axisof the drum member as possible. The upper bar 212 is arranged to locatethe axis of the writing head 218 precisely on the axis of the drum withthe axis of the writing head perpendicular to the drum axis. The upperbar 212 locates the translator in the vertical and the horizontaldirections with respect to the axis of the drum member. As illustratedin FIG. 9, the translator bearing 215 for the upper bar contacts onlythe upper surface of that bar as an inverted "V" to provide only thatconstraint to the translator, and is held in place by the weight of thetranslator. The lower bar 214 locates the translator member only withrespect to rotation of the translator about the bar 212 so that there isno over-constraint of the translator which might cause it to bind,chatter, or otherwise impart undesirable vibration to the writing headduring the generation of an image. This is accomplished by the lowerbearing 217 which engages the lower bar only on diametrically oppositesides of the bar on a line which is perpendicular to a line connectingthe centerlines of the upper bar and the lower bar. It has been foundthat such sliding bearings are superior for this application becausethey do not impart periodic motion to the translator assembly, and thusto the write head and the image generated thereby, which is oftenvisible in the resulting image as banding that is readily percieved bythe eye. Ball or roller bearings, even of high precision, are known tocause such periodic vibrations, which grow progressively worse as thebearings age. With the bearings used in the preferred embodiment, asthey age, they merely cause the optical axis of the write head to shiftslightly with respect to the drum axis which will not be perceived inany single image. Furthermore, such sliding bearings are toleranced suchthat they cannot be made too tight so as to bind and cause jitter in thetranslator that would degrade the quality of the image generatedtherewith.

Translator Drive

Referring now to FIGS. 8 and 9, the translator member 210 is driven bymeans of a motor 282 which rotates a lead screw 284 parallel to bars 212and 214 to move the writing head parallel with the axis of the drummember. The coupling 286 (see FIG. 8), which connects the drive nut onthe lead screw to the translator member, is arranged to accomodatemisalignment of the drive nut and the lead screw so that only forcesparallel to the linear lead screw and rotational forces are imparted tothe translator by the lead screw and the drive nut. Moreover, thecoupling is connected to the translator by means of a split sleeve thatis freely movable over the coupling and within the bore of thetranslator member until it is fixed by means of a set screw that clampsthe sleeve to both the translator and to the coupling. In this way, itis possible to release the set screw and remove the sleeve from thetranslator and the coupling, and even from around the lead screw, sothat the lead screw can be withdrawn from the translator and from theframe member for repair or replacement without the need to remove thetranslator from the apparatus.

The writing head 218 is removably mounted in V-shaped beds 288 which areformed in the translator member 210 in precise relationship with thebearings 215 for the upper bar 212 so that it automatically adopts thepreferred orientation with respect to the drum axis noted above. Thewriting head is selectively locatable with respect to the translator,and thus with respect to the drum surface, with regard to its distancefrom the drum surface, and with respect to its angular position aboutits own axis. Accordingly, a pair of adjustable locating means areprovided to accurately locate the writing head with respect to these twoaxes on the translator member 210. Only one of the adjustable locatingmeans, for example a micrometer adjustment screw 220, is illustrated. Anextension spring 223 (FIG. 9) is provided to load the writing headagainst these locating means.

The translator drive lead screw 284 is disposed between the two sides ofthe machine frame 204 where it is supported by bearings 292. At thedrive end, the lead screw shaft 293 continues through the bearing,through a pair of spring retainers 294 separated and loaded by acompression spring 296 to one side of an Oldham coupling 298. One of thespring retainers bears on the outer surface of bearing 292, which has asliding fit within the bore in the frame member 204, to axially locatethe bearing therein. The other spring retainer is located by a leadscrew retainer 300 attached to the outer surface of the frame member.The motor 282 is also mounted on the outer surface of frame 204 andincludes a through shaft 302, one end of which is attached to the secondhalf of the Oldham coupling 298, and the opposite end of which extendsthrough the flywheel 304 to a clutch 306 and to a shaft rotation encoder308. The flywheel is only connected to the shaft through the clutch. Theopposite end of the lead screw extends through bearing 292 to a wobblerod 310 and an adjustment screw 311 threadedly mounted in an end plate312 which is attached to the outer face of the frame member 204. Thewobble rod is provided with spherical sockets at each end that mate withspherical end surfaces, or balls 314, rigidly mounted in the ends of thelead screw and the adjustment screw.

In the translator drive arrangement the flywheel 304 is attached to themotor output shaft 302 through the clutch 306 and functions to minimizeany rotational flutter which might otherwise be imparted to the leadscrew by the motor. The flywheel is attached by means of the clutch sothat, should the mechanism be suddenly stopped by some malfunction inthe writehead assembly or some other portion of the transport, theinertia of the flywheel will not be imparted to the assembly andpossibly cause additional damage some portion thereof, since the clutchwould release under these conditions. The Oldham coupling 298accomodates any slight misalignment between the motor mount and the leadscrew mount while giving a stiff torsional connection between the two.The use of the wobble rod 310 improves the accuracy and stability of thelead screw against movement in the axial direction. It has been foundthat the stability and accuracy of such a wobble rod connection issignificantly better than that obtainable with thrust bearings. Anyvariation in the axial location and stability of the lead screw directlyaffects the accuracy of the placement of the image on the rotating drum.The compression spring 296 loads the lead screw axially against thewobble rod and the adjustment screw 311 and assures that all backlash isremoved from the system, while the adjustment screw permits theadjustment of the clearance in the Oldham coupling. Further, the Oldhamcoupling and the removable end cap 312 permits the removal andreplacement of the lead screw without the disassembly of other leadscrew drive components or the removal of the writehead assembly andtranslator.

Writing Head Assembly

FIG. 7 is a cross section of the writing head 218 which comprises agenerally cylindrical barrel portion 228 having a flange 230 at the drumend thereof. The interior of the barrel portion is arranged to accept astationary lens barrel 232 at the writing end, containing a stationarylens 234. A printhead assembly 236 is selectively oriented within and atthe opposite end of the barrel from the writing end. The printheadassembly comprises a tubular member selectively oriented within barrelportion 228 and contains a linear array of optical fibers which includesa fiber-supporting wafer or substrate 238 having a plurality of opticalfibers 240 mounted thereon. The optical fibers have a writing end 239facing the drum member 18 at the opposite end of the barrel. The opticalfibers 240 extend from the end of the printhead assembly and out of thewriting head barrel through a protective sheath 242 to the diode lasers206 (see FIG. 1).

A cup-shaped closure member 244 is arranged to mate with the flange 230of the writing head barrel 228 and forms a housing for the focusingdrive means, as will be described hereinbelow. As disclosed in commonlyassigned, copending U.S. application Ser. No. 670,095, filed in thenames of Harrigan and Carson on Mar. 15, 1991, the end of the closuremember adjacent drum member 18 is provided with an axially disposedopening which is bridged by a pair of sheet flexure members, 246 and248, mounted at the outer periphery thereof by annular plate means 250and 252 to the closure member 244. The central portions of the sheetflexure members are mounted to a movable rigid cylindrical lens housing254 which contains moveable lens 256. A cylindrical bobbin 258 isdisposed around the end of stationary lens barrel 232 and is connectedto the moveable lens housing 254 via equally spaced arms 260 whichextend between the legs of the flexure members 250 and 252. A voice coil262 is wound about the cylindrical portion of the bobbin 258 and isconnected to a driving circuit, to be further described hereinbelow.

Also enclosed between the end closure 244 and flange 230 is a highpower, toroidal magnet 264 and an annular magnetic plate 266 which areboth disposed about and spaced from the end of stationary lens barrel232. The voice coil portion of the bobbin 258 is disposed in the gapbetween the inner circumference of plate 266 and the outer circumferenceof stationary lens barrel 232. The dimensions of the magnet, the annularplate, the stationary lens barrel, and the bobbin are such that thebobbin can move freely axially of the lens barrel. The bobbin issupported in the gap by its attachment to the moveable lens housing 254which is held in position by the plate flexures 250 and 252. It will benoted that the barrel portion 228, flange 230, the stationary lensbarrel 232, and annular plate 266, are all formed of magnetic material,such as ordinary steel, so that in combination with the toroidal magnet264, a strong magnetic field is created between the inner periphery ofthe annular plate 266 and the end of the stationary lens barrel 232. Asa result, when a current is introduced into the voice coil 262 of thebobbin 258, as by a lens focusing circuit (not shown), an axial force isimparted to the bobbin and to the moveable lens housing 254, therebyselectively moving the moveable lens 256 along the optical axis of theassembly. Thus, with an appropriate focus detection system, to bedescribed hereinbelow, the moveable lens assembly may be driven toassure that the output of the fiber optic array is maintained in focusat the appropriate position on the imaging drum 18, or on or within thewriting element (not shown) mounted thereon.

The fiber optic array comprises a plurality of fibers 240 which are eachconnected to a respective, remotely mounted diode laser 206. The diodelasers can be individually modulated to selectively project light fromthe writing end 239 of the optical fibers through the lens assembly,consisting of stationary lens 234 and movable lens 256, onto the thermalprint medium carried by the imaging drum 18. The fiber optic array maybe of the type shown in copending, commonly assigned U.S. applicationSer. No. 451,656, filed Dec. 18, 1989, and comprises optical fibers 240which are supported on the substrate 238. Each of the optical fibersincludes a jacket, a cladding, and a core, as is well known in the art.As disclosed in the copending application, the fibers extend from thelaser diodes to the array and are mounted in sets of grooves (not shown)which are formed in the substrate so that the fibers at the writing end239 are disposed substantially parallel and adjacent to each other invery close proximity, with the ends disposed in a common planeperpendicular to the fiber axes. In the preferred embodiment of thearray, twenty writing fibers 240 are employed. As illustrated incommonly assigned, copending U. S. application Ser. No. 670,095, thesubstrate 238 is disposed in the tubular member of the printheadassembly 236, with the tubular member being provided with a keyway whichmates with a corresponding key (not shown) on the inner surface ofbarrel portion 228 so that the orientation of the linear array 240 is ata preselected angle with respect to the drum axis 202.

The end of the writing head 218 adjacent the imaging drum 18 is providedwith a pair of photosensors 226 aimed at the surface of the drum member.The photosensors may each include an infrared source or they may rely onan outside source of light energy. The photosensors are disposed ondiametrically opposite sides of the optical axis of the writing head ina fixed relationship thereto. The orientation of the keyway in the outersurface of the printhead assembly 236, the corresponding key on theinterior of the barrel portion 228, and the photosensors 226 disposed ondiametrically opposite sides of the writing head axis, all correspond sothat when the two photosensors 226 are exactly parallel with the axis202 of imaging drum 18, the writing angle of the linear array 240 isthat which has been preselected for the present apparatus.

The determination of this relationship is relatively simply achievedwith the present construction inasmuch as a visible line 268 is providedon the drum surface which is carefully fabricated to be parallel withthe drum axis. Accordingly, when the photosensors 226 both detect line268 simultaneously, the writing head has the proper angular orientationto provide the desired angle of the linear array with respect to thedrum axis. Adjustment of the angular positioning of the writing head isequally easy to obtain. Hold down clamps 270, which lock the writinghead 218 on the translator member 210, are loosened, and the micrometeradjustment screw 220 is adjusted against a stop on the translator memberto rotate the head member against the force of the torsion spring 222,or to permit the torsion spring to rotate the writing head in theopposite direction, should that be necessary. When the photosensors 226both simultaneously detect line 268, which may be accomplished when thedrum is either moving or stationary, with or without the writing elementdisposed thereon, the desired angle between the linear array and thedrum axis is achieved. With this construction it is possible to replacethe writing head in the field with a new writing head without requiringelaborate setup or alignment, since the predetermined relationship hasalready been established between the photosensors 226 and the lineararray when the writing head is assembled.

The diode laser 206 are individually disposed in a unique mount 207which is illustrated in both assembled and exploded form in FIGS. 24 and25. The mount includes a heat sink 209 onto which the diode laser 206 ismounted. A bracket 211 is also mounted to the heat sink and includes aholder 213 for a fiber optic connector 215. An optical fiber guide andmode mixer 217 is also mounted on the bracket 211 and is held in placeby a cover plate 219. The optical fiber guide and mode mixer 217 isformed either of a rigid molded plastic or an open cell foam plastic andis provided with an inlet channel 221, a cylindrical chamber 223, and anoutlet chamber 225. The optical fiber 227 is conneced to the laser diode206 and enters the fiber guide 217 through the entry chamber 221. Thefiber is wound in chamber 223 until substantially all the excess lengththereof is accomodated and then the fiber is routed through exit chamber225 so that the fiber connector 215 can be mounted rigidly in the holderportion 213 of bracket 211. The cover plate 219 is provided with arestricted access channel 229 therethrough which generally follows thepath of the fiber in the fiber guide. With this construction, asubstantial length of fiber may be wound into the cylindrical chamber223, providing mode mixing for the light passing therethrough andaccomodating variations in the length of the fiber between the laserdiode and the outlet connector 215. Moreover, any odd length of fibernot accomodated by a full wrap within the cylindrical chamber 223 may beaccomodated by the outlet chamber 225. The access channel 229 throughthe cover plate 219 permits the insertion or withdrawal of the fiberinto or from the fiber guide without disassembly, but is sufficientlynarrower than the chambers in the fiber guide that it retains the fiberin the guide unless the fiber is deliberately pulled through the accesschannel. Accordingly the present invention provides a compact laserdiode and optical fiber mount and guide assembly wherein the fiber isguided and held relatively stationary while providing the mode mixingnecessary to even out the light transmitted thereby, and at the sametime accomodating a variable length of fiber with no modifications tothe fiber necessary.

The focus detection system comprises a second array of optical fibers271 mounted on the opposite surface of the substrate 238 with respect tothe writing array 240. The focusing array 271 requires only a singlefiber but, in practice, three fibers may be provided, with two as extrasin case the first fiber fails. The focusing fiber is connected at itsinlet end to a focusing laser diode, the housing 290 for which ismounted on the writehead assembly. In prior arrangements, the focusinglaser diode was stationarily mounted remotely from the write head withthe laser diodes for the writing array, and connected thereto by anoptical fiber which permitted relative motion between the moving writehead and the stationary laser diodes. However, it was found that themovement of the focusing optical fiber as the write head traversed theimaging drum generated sufficient noise in the light beam used for thefocusing system, that the focus signal sensed by the photodetector wastoo variable to accurately control the focus of the writing array. Ithas been found that by mounting the focusing laser diode on the writehead, with the optical fiber connecting the diode to the write headremaining substantially stationary despite the motion of the write head,that the clarity of the focus signal is significantly enhanced. Thefocusing diode housing 290 is not shown in FIG. 6 for the sake ofclarity, but is illustrated in FIGS. 8, 9, and 10.

The focusing laser diode is selected to produce a second beam of lighthaving a wavelength different from the wavelength of the writing beamand preferably outside the range of 800 nm-880 nm. Preferrably, thefocusing light source produces a beam of light having a predominantwavelength of 960 nm. It has been found that a focusing beam having awavelength of 960 nm is substantially unabsorbed by all of the variousdonor dye materials. As a result, substantially all of the focusing beamof this wavelength will penetrate the donor material, regardless of thecolor dye employed, to be reflected from the reflective surface which ispart of the receiver element. Inasmuch as this surface has been found tobe much closer to the dye layer, where it is desirable to focus thewriting beam, than the top surface of the donor layer, it is possiblefor both the writing beam and the focusing beam to be aimed at morenearly the same surface than is possible if the focusing beam isreflected from some other surface of the writing element. As a result,the writing beam may have less depth of focus and consequently may havea greater numerical aperture which permits the transmission of greaterwriting power to the writing element than would be the case were thefocusing beam and the writing beam to be focused at more widelyseparated surfaces.

With the mounting of the focusing laser diode on the write head it hasbeen necessary to provide an optical fiber spool to accomodate thelength of the optical fiber connecting the focus diode to the write headarray. This is necessary because a sufficiently long optical fiber mustbe provided for the focus diode that, after the one end of the fiber ismounted on the writing array substrate, the opposite end of the fibermay be polished for inclusion in an optical fiber coupling. Thus, theexcess length of fiber must be accomodated without permitting motion ofthe fiber which can reintroduce undesirable noise into the focus signal.A focus fiber spool is illustrated in FIG. 27, and comprises amodification of the write head 218 illustrated in FIG. 7. The tubularmember of the printhead assembly 236 is provided with an external flange241 at the end opposite from the lens 234, and an internal thread toengage a fiber spool 243 which has an internal fiber bore 245, a crossbore 247 for the focus fiber 271, and an external flange 249 which isarranged to be spaced from the flange 241. A sleeve 253, having a nipple255 arranged to mate with a focus fiber protective sheath 257, isdisposed between and held by the two flanges 241 and 249. To assemblethe spool, the writing fibers and the focus fiber are mounted on thesubstrate 238, and all of the fibers are routed through the bore of theprinthead assembly 236. The focus fiber is then threaded into the bore245 of the spool 243 and is fed out of the cross bore 247. The writingfibers are directed straight through the bore 245 of the spool, into theprotective sheath 242 and to an optical fiber coupler 259. The focusfiber is then fed through the nipple 255 in the sleeve, through theprotective sheath 257 to a coupler 261. The substrate is then mounted inthe tubular member of the printhead assembly. The spool is then threadedinto the end of the printhead assembly 236 with the sleeve held looselybetween the flanges 241 and 249. The sleeve 253, with the focus fiberextending through the nipple 255, is rotated between the flanges,wrapping the excess focus fiber around the outer surface of the spooluntil the excess length is taken up. This wrapping action has no effecton the writing fibers since they are free of the spool and do not becometwisted thereby. After all of the excess focus fiber has been wound onthe spool, the spool is tightened in the printhead assembly, locking thesleeve in place. Thus the excess focus fiber is accomodated andprevented from moving and degrading the focus signal.

As illustrated in FIG. 7, the focus detection system also includes abeam splitter 272, having a semi-reflective buried surface 274, which isdisposed between the writing end 239 of the linear array 240 and thestationary lens 234. A split cell photodetector 276 is disposed in thesidewall of barrel 228 and is arranged to receive the portion of thefocusing beam which is reflected from the writing element and by theburied layer of the beam splitter. A knife edge 278 is provided betweenthe beam splitter and the photodetector 276. It has been found that thequantity of light from the writing array reaching the photodetector,illustrated in phantom at 279 in FIG. 28, is sufficiently great that,despite the precautions noted above, it can overwhelm the focus signalat the photodetector. Accordingly, a micromask 280 is provided having anaperture 281 with a diameter in the range of 300 microns to 600 microns,and preferrably having a diameter of about 400 microns. This masksignificantly contributes to the ability of the system to prevent energyfrom the writing array from reaching the focusing photodetector. A maskhaving a thickness of approximately 400 microns, spaced from the surfaceof the photodetector a distance in the range of approximately 125microns to 500 microns, and preferrably at a distance of 400 microns,satisfactorily permits a reflected focus beam 283 with a diameter ofabout 50 microns to be utilized by the split-cell photodetector forcontrolling the focus of the writing head array in spite of the presenceof the reflected portion of the writing array, consisting of twentybeams of approximately 50 microns in diameter originally each having apower twenty times that of the focusing beam, impinging on the face ofthe photodetector mount only one millimeter away.

Thus, the focus detection system comprises the laser diode producing abeam of light having a wavelength of 960 nm, a focusing optical fiber271 which is disposed on the opposite of the mounting substrate from thelinear array of the writing beam, and which is arranged to project thefocusing beam through the beam splitter 272 and the focusing assemblycomprising lenses 234 and 256, which are illustrated as single lensesbut may comprise groups of simple or complex lenses. The focusing beamof light is then projected onto the drum surface, or the writing elementdisposed thereon, and is reflected from the reflective surface backthrough the focusing assembly and into the beam splitter 272 wherein aportion of the reflected focusing beam is deflected by the buried layerpast the knife edge 278 and through the aperture 281 in the mask 280onto the split cell photodetector 276. In the preferred embodiment, thephotodetector 276 has a preferential wavelength sensitivity to thewavelength of the focusing beam, i.e. 960 nm. The signal from thephotocell 276 is fed to a focusing circuit, not shown, which thengenerates an appropriate current which is supplied to the voice coil 262on the bobbin 258 attached to the movable lens element 256. In this waythe focus detection system constantly monitors the location of a surfaceclosely adjacent the surface of the writing element on which the writingbeam is to be concentrated.

It has been found that the initial coarse focus of the system can befacilitated by the use of the present focusing arrangement. It isnecessary to approximate the final focus in the initial system set-upbecause it has been found that, without such an initial set-up, thewriting array may be so far out of focus that the reflection of thewriting beams from the surface of the writing element or from theimaging drum surface creates such a large, out-of-focus image at thefocus photodetector that it will spill over into the focus beam area,flooding the focus beam photodetector to the extent that there would beno hope of establishing a true focus thereof. Accordingly, provision ismade to operate the focus system alone, without operating the writingbeam array, while defeating the operation of the voice coil drive to theauto-focus lens. Thus the installer of the writehead assembly initiallysets the lens position by axially locating the barrel 228 in thetranslator 210 by reading the output of the split-cell photodetector 276with a suitable measuring device such as an ocilloscope or a voltmeter.As the writehead assembly is moved toward the desired focal point, theoutput of the photodetector will rise to a maximum and then will fallthrough zero to a miminum. The desired focal setting for the initialfocus of the writehead assembly and the auto-focus system is the pointwhere the output crosses zero, and the assembly is clamped to thetranslator at that position.

Imaging Drum

Details of the imaging drum 18 and its drive are illustrated in FIGS.12-15. The drum is generally hollow and comprises a cylindrical shell320 which is provided with a plurality of vacuum perforations 322therethrough. The ends of the drum are closed by cylindrical plates 324and 326 each of which is provided with a centrally disposed hub, 328 and330, which extend outwardly therefrom through support bearings 332 and334, respectively, in the respective portions of the frame member 204.Hub member 328 extends through the bearing 332 and is stepped down torecieve a motor armature 336 which is held on to the hub by a fastenernut 338. A motor stator 340 is stationarily held by the frame member204, encircling the armature 336 to form a reversible, variable speeddrive motor for the imaging drum. The outer end of hub 328 is connectedto an encoder 342 which is supported from the frame member 204 and iscoupled to the hub by means of a flexible coupling 344. The opposite hub330 is provided with a central vacuum opening 346 that is in alignmentwith a vacuum pipe 348 which is provided with an external flange 350that is rigidly connected to the outer surface of frame member 204. Thevacuum pipe is provided with an extension 353 which extends within butis closely spaced from the internal surface 346 of hub 330. With thisarrangement, a slight vacuum leakage channel is provided between theouter diameter of the vacuum pipe extension 353 and the inner diameterof the vacuum opening in the hub 330 which assures that no contactexists between these parts which might impart uneven movement or jitterto the imaging drum during its rotation. The opposite end of the vacuumpipe is connected to a high-volume vacuum pump which, in the preferredembodiment, is capable of generating a vacuum of 50-60 inches of H₂ O ata volume of 60-70 cfm.

The outer surface of the imaging drum is provided with an axiallyextending "flat" 352 which extends approximately 8 degrees of the drumcircumferance and includes the axial indicia line 268, substantially asillustrated in FIG. 13. The drum is also provided with a circumferentialrecess 354 which extends circumferentially of the drum from one side ofthe flat 352 around the drum to the other side of the flat, and fromjust outside the inner wall of end plate 326 to just outside the innerwall of end plate 324, substantially as illustrated in FIG. 12. Thecircumferential recess is exaggerated in the illustration since thedepth thereof is substantially equal to the thickness of the receiversheet which is to be seated therein, e.g. approximately 0.004 inches inthickness. The end plates are provided with circumferential grooves 356which are in communication, for the most part, with the interior of thedrum and act to provide vacuum to the circumferential ends of the drumsurface. A view of the drum surface generated by the unrolling thereofis illustrated in FIG. 15, with the surface divided axially along theline 268 which is also the centerline of the surface flat 352. Theopposite edges of the drum flat are illustrated at the dotted lines352a. The axial edges of the recess 354 are illustrated at 354a. Thearea covered by the receiver sheet adhered to the imaging drum surfaceis indicated by dotted lines 358, which area extends axially almost fromone side of the recess 354 to almost the other and circumferentially ofthe drum from one edge of the flat to the other, without overlapping it.

A valve block 360 is disposed axially along the inner surface of thedrum substantially beneath the flat 352 on the outer surface thereof andprovides two separately controllable vacuum chambers 362 and 364 beneathselected vacuum holes disposed on either side of the axial centerline268 of the drum flat 352. The first chamber 362 extends along theplurality of vacuum openings disposed in the "leading edge" of the drumflat 352. The term "leading edge" refers to the portion of the drum flatonto which the leading edge of the donor material is disposed when it issuperposed over the receiver sheet and the drum is spun in the normalwriting direction as indicated by arrow 365 in FIG. 13. This chambercontrols the hold-down of the leading end of the donor sheet, as will bemore thoroughly described hereinbelow, and will be referred to as the"donor vacuum chamber" hereafter. The extent of the donor vacuum chamber362 is illustrated by lines 362a in FIG. 15 and will be seen to extendinto a portion of the chambers 356 which are provided along thecircumferential edge of the drum. The second, separately controllablevacuum chamber 364 extends along the trailing edge of the receiver sheetwhen it is mounted on the drum, as indicated by dotted lines 364a inFIG. 15, and will be referred to as the "receiver vacuum chamber"hereafter. The imaging drum is also provided with a counterbalance block366 opposite to the vacuum chamber block 360 in order to provide dynamicbalance to the drum. Each of the balancing block 366 and valve block 360are provided with passageways 370 which permit vacuum to be applied tovacuum holes that would otherwise be covered by the respective blocksand which are not intended to be controlled by the vacuum chambers 362and 364. The area of the drum circumference covered by the donor sheetwhen it is superposed with the receiver sheet is indicated by lines 371in FIG. 15. Further, a registration indicia 373 is disposedsubstantially axially centrally of the drum and centered over the centerline 268 of the axial flat 352, the purpose of which will be describedhereinbelow.

The purpose of the drum flat 352 is twofold; it assures that the leadingand trailing ends of the donor sheets are somewhat protected from theeffect of the air during the relatively high speed drum rotation duringthe writing process. Thus the air will have less tendency to lift theleading or trailing end of the donor sheets. The drum flat also assuresthat the leading and trailing end edges of the donor sheet are recessedfrom the drum periphery so that there is less chance that they can comeinto contact with other parts of the apparatus such as the end of thewriting head and cause damage. The drum flat also acts to impart abending force to the ends of the donor sheets when they are held to thedrum surface by the vacuum within the drum so that, when that portion ofthe vacuum is turned off, that end of the donor sheet will tend to liftfrom the drum surface by the release of the bending force on the sheet.This is used to advantage in the removal of the donor sheet from theimaging drum, as will be described hereinbelow.

The purpose of the recess 354 in the imaging drum surface is toeliminate any creases in the donor sheets as they are drawn down overthe receiver sheet during the loading of the donor sheet. This assuresthat no folds or creases will be generated in the donor sheet whichcould extend into the image area and seriously adversely affect theresulting image. The recess also substantially eliminates theentrainment of air along the edge of the receiver sheet where it isdifficult for the vacuum holes in the drum surface to assure the removalof all the air. Any residual air between the receiver sheet and thedonor sheet can also adversely affect the image.

Controlled Vacuum Chamber and Control Valve

An enlarged detail of a controlled vacuum chamber and the control valvetherefor is illustrated in FIG. 14 wherein the receiver vacuum chamber364 is illustrated. As illustrated, the vacuum chamber 364 communicatewith the central interior vacuum chamber 363 of the drum via passageways372 in each of the end plates 324 and 326. The control valve comprises avalve actuator 374 which extends outwardly from the end plate 326 and isarranged for engagment by an actuator cam 388 which is illustrated inFIGS. 16-19 and will be described more throughly hereinbelow. The valveactuator 374 is connected internally of the vacuum chamber via anelongated rod 376 to first and second valve members 378 and 380 atopposite ends of the valve chamber 364. Each of the valve members isarranged to mate with a cooperating seat which closes the respectivepassage 372, blocking the vacuum in the main drum chamber 363 from beingtransferred to the controlled vacuum chamber 364. At the same time thatvalve member 378 closes the passage 372, it also opens the valve chamber364 to the atmosphere through opening 382 in the end wall 326 of thedrum (see FIG. 17). Valve member 380 is spring loaded against the end ofthe rod 376 so that it can accomodate slight differences between thelength of the rod and the length of the chamber. The rod 376 is providedwith a spring retainer 384 which abuts a shoulder in the wall of thechamber 364 to load a spring 386 which returns the valve to the positionwhere the chamber is open to the main vacuum chamber 363 when the valveactuator is released by the valve actuator cam 388 (see FIG. 18).

The vacuum chamber valve actuator cam 388 is illustrated in FIGS. 16-19and comprises a generally vertically mounted planar member which ispivoted from a support, not shown, from its upper end about a pivot 390.The valve actuator cam is mounted from the bearing cap 392 at the vacuumsupply end of the imaging drum as illustrated in FIG. 11. A portion ofthe bearing cap 392, partially in section, is illustrated in FIGS. 17and 18 and is provided with a recess 394 in the face thereof facing theend of the drum 18 to receive the end of the valve actuating cam 388when it is in its "at rest" position, as illustrated in FIG. 18. Theface of the cam is provided with actuator surfaces 396 and 398 which arearranged to engage the actuators 374 of one or both of the controlledvacuum chambers 362 and 364. The cam face is also provided with a recess400 that permits the cam to not operate the valve for the receiverchamber 364 during the operation of the valve for the donor chamber 362at one stage of the operation, as described below. The cam is alsoprovided with ramping surfaces 402 to prevent damage to the valveactuators should the drum be inadvertantly rotated while the actuatorcam is in the operative position illustrated in FIG. 17. The actuatorcam 388 is driven by means of a rotory cam 404 which engages a camextention 406 on the back surface of actuator cam 388. The rotary cam404 is mounted on a shaft and is driven by a motor 408, shown in FIG.11, mounted on the outer surface of the bearing cap 392.

Exit Transport

The waste sheet exit transport 22 comprises a donor stripper blade 410disposed adjacent the upper surface of the imaging drum and movablebetween an unloading position wherein it is in contact with the sheetson the drum surface and an inoperative position wherein it is moved upand away from the surface of the drum. A driven waste transport belt 412is arranged substantially horizontally to carry used donor sheetsremoved by the stripper blade 410 to an exit 414 from the machine.

The image sheet exit transport 24 comprises a stationary image exitblade 416 disposed adjacent the top surface of the imaging drum 18substantially opposite from the movable stripper blade 410. An imagesheet transfer belt 418 is arranged for cooperation with a vacuum table420 to deliver a receiver sheet with an image formed thereon to an exittray 422 in the exterior of the apparatus.

Air Flow

The location of the various elements within the apparatus cabinet 26 hasbeen chosen so that, along with the airflow to be described below, theoverall operation of the apparatus is enhanced. Specifically, thelocation of the material supply assembly 12 and the sheet cutterassembly 14 have been selected so that any dust introduced into theapparatus along with the material supply rolls, or generated by thesheet cutter are as remote as possible from the image writing location.It has been found that the quality of the image may be adverselyaffected if dust is permitted to come between the donor sheet and thereceiver sheet when they are superposed on the imaging drum.Accordingly, since the sheet cutting assembly 14 is known as a dust anddirt generating site, it is placed in the lowermost location within theassembly cabinet. Further, an exhaust fan 82 is arranged to draw air infrom the general cabinet area, through the cutter assembly 14 and toexhaust this potentially dirt-laden air through an exit 424 from theapparatus cabinet.

The apparatus cabinet is generally sealed against entry of air so thatair may be introduced into the cabinet through an air inlet 426 via avery fine filter assembly 428 by means of a forced air fan 430. The airfrom this fan is conducted by a duct 432 and the flow is indicatedgenerally by the open arrows in FIG. 26. A portion of the air issupplied to the air chamber 90 of the sheet transport assembly 16 and iscontrolled by the damper valve 93. A further portion of the air isgenerally directed over the sheet transport assembly. Further downstreamair is ducted into the imaging assembly area where it passes over theimaging drum and the writehead assembly to be exhausted through thewaste and image exits. The air at the imaging assembly area is at a highenough pressure compared to the other interior portions of the cabinetthat it also flows from the imaging assembly area into the sheettransport assembly area to preclude any dirt carrying air from that areafrom entering the imaging area. The air from the sheet transport area isexhausted through the sheet cutter assembly by the exhaust fan 82, asnoted above. Another portion of the air is directed at the circuitboards and other electronic components, generally indicated at 434, tocool these devices before it exits from the apparatus through an exit436. A further portion of air is ducted to the laser diodes 206 to coolthem before passing to an exit 438 from the apparatus.

Accordingly, it will be seen that with the present apparatus, the airflow therethrough is carefully controlled to minimize the possibility ofdust and dirt being conducted into the imaging area and thuscontaminating the sheet elements and adversely affecting the imagegenerated therein. Moreover, the locations of the components supplyingmaterials to the imaging station have been chosen to place the dirt anddust generators as far from the imaging area as possible, and to utilizegravity to further diminish the transfer of dirt to the imaging area.Still further, the use of an exhaust fan to exhausting air from thecabinet through the sheet cutter assembly also contributes to thecleanliness of the imaging area.

Operational Sequence

Referring now to FIGS. 19-23, the operational sequence of the laserthermal printer proofer of the present invention will be described. Thesequence step order is indicated at the beginning of each step as anumber in a bracket [#] which also corresponds to the sequence numberindicated in the chart illustrated in FIG. 23. The drum centerlinepositions noted in FIG. 23 are illustrated schematically in FIG. 20. [1]With the imaging drum 18 located with the centerline 268 of the drumflat 352 located in the "Home" position, i.e. directly in line with theoptical axis of the writehead assembly 218, the following is the statusof the various components and assemblies of the apparatus: the carousel30 is stationary, the imaging drum 18 is stationary, as are the meteringroll 86 and the sheet transport rolls 102 and 104. No material issuperposed on the imaging drum and the vacuum pump connected thereto isoff. The squeegee roller 97 is disengaged from the imaging drum as isthe donor stripper blade 410. To start, the carousel is rotated untilthe supply roll 44 of the receiver material is located adjacent thesheet cutter assembly 14. The carousel is stopped and the edge guide 96is disposed in the first, receiver, position. The air to the air table92 is turned off by closing the damper valve 93. The sheet feed roll 48is driven by the drive 72, as previously described, feeding the end ofthe receiver web into the sheet cutter assembly where it is engaged bythe metering roll 86 and belt 88 and advanced until the proper length ofmaterial is determined. The cutter blades 84 are actuated. The conditionof the overall system at this point is illustrated in FIG. 22a.

[2] The imaging drum is then rotated clockwise and is stopped with thecenterline 268 of the drum flat 352 disposed under the sheet sensor 101at position H (see FIGS. 20 and 23). All other conditions are the sameas step 1. With the centerline 268 disposed at position H, the sheetsensor 101 does not see the non-reflective registration indicia 373 (seeFIG. 15). [3] The sheet transport assembly 16 is then actuated and thereceiver sheet is driven up until the leading edge thereof is sensed bythe sheet sensor 101 and the transport assembly drive rollers and thesheet are stopped. As the receiver sheet is driven into engagement withthe drum, the sheet sensor detects the lead edge of the sheet by seeingthe reflection from the surface of the sheet and stops the sheet withthe edge at the sheet sensor 101 centerline at position A. [4] Theimaging drum is then rotated counterclockwise to the receiver loadposition "B" (see FIG. 20) and is stopped. [5] The vacuum to the imagingdrum is then actuated by actuating the vacuum pump. All of the vacuumchambers in the imaging drum are supplied with vacuum at this point, asindicated in FIG. 21a wherein all of the vacuum openings are suppliedwith vacuum. [6] The load or squeegee roller 97 is moved into engagementwith the end of the receiver sheet, pressing it into contact with theimaging drum where the vacuum operates to hold it to the drum surface.The condition of the overall system at this point is illustrated in FIG.22b. [7] The imaging drum is then rotated counterclockwise until thetrailing edge of the receiver sheet is under the squeegee roller 97. Thesqueegee roller acts to facilitate the removal of all of the air frombetween the receiver sheet and the drum surface. [8] The imaging drum isthen rotated clockwise until the lead edge of the receiver sheet isagain beneath the squeegee roller 97. The air to the air table 92 isturned on by opening the damper valve 93. The condition of the overallsystem at this point is illustrated in FIG. 22c. [9] The load roller 97is moved to the disengaged position, spaced from the surface of theimaging drum.

[10] The carousel is rotated and stopped at the appropriate donor supplylocation, and the edge guide 96 is moved to the second, donor positionso the donor sheet will properly overlap the receiver sheet when thedonor sheet is superposed therewithon the imaging drum. The sheet feedroll 48 is driven by the drive 72, as previously described, feeding theend of the donor web into the sheet cutter assembly where it is engagedby the metering roll 86 and belt 88 and advanced until the proper lengthis reached. The cutter blades 89 are actuated to cut a donor sheet fromthe donor web. The imaging drum is rotated clockwise to the donor sheetloading position "H". [11] The valve actuator cam 388 is actuated bymotor 408 operating through the cam 404 to move the valve actuator caminto the operative position illustrated in FIGS. 17 and 19a, wherebythe-cam surface 396 engages the valve actuator 374 "DC" for the donorchamber 362, closing the passages 372, turning off the vacuum to thatchamber and opening it to atmospheric pressure. This is necessary atthis point because, since the receiver sheet has previously beensuperposed on the majority of the drum surface, closing off the majorityof the vacuum openings therethrough, the vacuum now available at thelead edge of the donor sheet is sufficiently strong that it mightprevent the movement of the sheet over the drum surface were thatportion of the vacuum holes not be isolated from the vacuum. Referringto FIG. 19a, it is seen that the valve actuator 374 "R" for the receiverchamber 364 is located in the recess 400 of the valve actuator cam 388and is not acted upon thereby. This condition of the vacuum chambers isillustrated in FIG. 21b wherein the donor vacuum chamber 362 iscrosshatched to indicate that no vacuum is being applied thereto, butthe remainder of the vacuum holes are supplied with vacuum.

[12] The donor sheet, which has previously been fed from the appropriatesupply roll on the carousel, cut and supplied to the sheet transportassembly is now located on the sheet transport assembly 16 and is drivenupward and stopped with the leading edge of the donor sheet at the sheetsensor 101. The sheet sensor 101 has previously checked the location ofthe receiver sheet to assure that it does not overlie the registrationindicia 373 and, if it does, to generate a fault signal to stop thesequence. The sensor now senses the lead edge of the donor sheet toassure that it overlies the proper area of the registration indicia asindicated in FIG. 15. The condition of the overall system at this pointis illustrated in FIG. 22d. [13] The valve actuation cam 388 is thendisengaged, releasing the donor vacuum chamber valve to reapply vacuumto the donor vacuum chamber 362 and thus to the leading edge of thedonor sheet. [14] The loading squeegee roller 97 is then moved to theengaged position forcing the donor sheet into engagement with the drumflat 352. [15] The imaging drum is rotated counterclockwise until thetrailing edge of the donor sheet is under the squeegee roller. Thecondition of the overall system at this point is illustrated in FIG.22e. [16] The imaging drum is reversed and rotated clockwise until theleading edge of the donor sheet is again under the load roller. It hasbeen found that without reversing the imaging drum and re-rolling thedonor sheet into contact with the receiver sheet that, although itappears that all air has been removed from between the sheets that someair can still remain before the second pass of the squeegee roller. Suchentrained air is unnoticed until the image has been formed when areas oflow density are found to have occurred because of entrained air. Withthe second pass of the squeegee roller pressing the donor sheet intocontact with the receiver sheet, such unnoticed entrained air iseliminated. At this point the sheet sensor again checks for the properpositioning of the donor sheet and, if the trailing end edge of thedonor sheet overlies the proper area of the registration indicia,indicates both that the donor sheet is properly located and that it isof the proper length. If the sensor detects the non-reflectiveregistration indicia when it should sense the donor surface, an errorsignal is generated, interrupting the sequence. [17] The squeegee rolleris disengaged from the imaging drum and the imaging drum is acceleratedin the counterclockwise direction to the image writing speed. At thistime the imaging drum and the writehead translator are sychronized bymeans of the encoders associated with each drive and the imaging processcommences.

After the image has been written onto the receiver sheet from the firstdonor sheet, the first donor sheet must be removed from superpositionwith the receiver sheet without moving the reciever from its location onthe imaging drum surface, since the registration of the multipass imageis determined by assuring that the position of the receiver sheetremains the same for the multiple writing steps. As a result, it isimportant that the donor sheet be removed without disturbing thereceiver sheet. This is accomplished by the following sequence of steps:[21] The imaging drum is stopped at the donor unload position indicatedby position "F" in FIG. 20. [22] The donor stripper blade 410 isactuated to the position against the imaging drum surface. [23] Thevacuum chamber valve actuator cam 388 is actuated to engage the valveactuators to both the donor vacuum chamber 362 and the receiver vacuumchamber 364. The location of the valve actuators with respect to theactuator cam surface 398 is illustrated in FIG. 19b along with thelocation of the valve actuators as they are moved toward the followingstep. The condition of the vacuum chambers is illustrated in FIG. 21cwherein both chambers 362 and 364 are open to atmosphere, as indicatedby the shaded portion. At this point, since the vacuum under the leadingedge of the donor sheet has been turned off, and since this portion ofthe donor sheet has been wrapped around the leading edge of the drumflat, the beam strength of the sheet material tends to lift the leadingedge of the donor sheet from the drum flat as illustrated in FIG. 22f.It will be appreciated that, although the vacuum to the trailing edge ofthe receiver sheet has also been turned off, the trailing edge of thereceiver sheet will not lift away from the surface of the imaging drumbecause it is still held by the superposed trailing edge of the donorsheet which overlies the trailing edge of the receiver sheet and is heldby the vacuum holes in the trailing edge of the drum flat which arestill being supplied with vacuum. Both valve actuators are actuated atthis time primarily to simplify the vacuum actuator cam geometry, andbecause no disadvantage is encurred thereby. [24] The imaging drum isnow rotated counterclockwise until approximately 1/4 inch of the leadingedge of the donor sheet, which has raised up away from the flat on thesurface of the imaging drum, engages the donor stripper blade,substantially as illustrated in FIG. 22g. [25] The donor stripper bladeis then moved to its disengaged position, with the leading edge of thedonor sheet supported thereon. The donor exit drive belt 412 isenergized at this point. [26] The valve actuator cam is moved to itsinoperative position, illustrated in FIG. 18, reapplying vacuum to allof the vacuum chambers in the drum. [27] The imaging drum is rotatedcounterclockwise to completely strip the donor sheet from superpositionwith the receiver sheet and to drive it to the waste exit 414 of theapparatus.

The imaging drum is now ready for the superposition of the next donorsheet with the receiver material already registered thereon andcontaining a first image recorded from the first donor sheet. The seconddonor is then loaded onto the imaging drum by repeating steps [9-17]from the above loading sequence, and the next image is written onto thereceiver. That donor is then removed according to the foregoingunloading sequence of steps [21-27]. This sequence continues, utilizingas many donor material sheets as the operator or program calls for. Theapparatus is then ready to unload the receiver sheet bearing thefinished image.

To unload the finished receiver, the following sequence is employed:[31] The imaging drum is stopped at the receiver unload position "D".[32] The valve actuator cam 388 is actuated engaging both of the valveactuators 374 for vacuum chambers 362 and 364. This releases thetrailing end of the receiver sheet, which is no longer held down by asuperposed donor sheet, and the receiver exit transport belt 418 andvacuum 420 are activated. The condition of the overall system at thispoint is illustrated in FIG. 22h. [33] The imaging drum is rotatedclockwise until the trailing end edge of the receiver sheet is engagedby and lifted from the drum by the receiver sheet exit guide 416. Thecondition of the overall system at this point is illustrated in FIG.22i. [34] The valve actuators are disengaged by deactivating the valveactuating cam 388, permitting vacuum to be reapplied to all of the drum.[35] The drum is rotated clockwise driving the receiver sheet onto thereceiver sheet exit guide onto the receiver exit transport belt 418.Even though the vacuum has been reapplied to the imaging drum, inasmuchas the receiver sheet is being peeled from the surface of the drum bythe receiver sheet exit guide, the number of vacuum holes open to theatmosphere is progressively increasing as the receiver sheet is removed,so that less and less vacuum hold down is provided to the receiver sheetremaining on the imaging drum, with only an amount of vacuum remainingsufficient to retain the "leading" end of the receiver sheet in positionuntil the drum has rotated sufficiently that the entire receiver sheethas been removed therefrom. The finished receiver sheet is exited fromthe machine to be used or laminated to a paper stock for use as a proof.[36] The imaging drum is then rotated counterclockwise to the " Home"position, the vacuum is turned off and the apparatus is ready togenerate the next proof.

It will be understood that should nonusable leader material be providedfor either the receiver material supply roll or the donor supply rolls,it will be initially discarded by being cut from the lead end of thesupply rolls and transported out of the apparatus via the waste exittransport 412.

Alternative Embodiments

While the preferred embodiment has been described with respect toapparatus that employs a rotating imaging drum, many of the features andadvantages thereof can be incorporated in a method and apparatusemploying a driven platen to carry the superposed receiver and donormaterials. While certain preferred operating conditions and ranges havebeen set forth above, it will be understood that the apparatus can useother operating conditions and ranges. For example, the writing laserdiodes may operate with a variable power range of 160-500 mw each, atwavelengths in the range of 800-880 nm. The writing beam may have a spotsize at the writing plane of 18-30 um, generated by optical fibershaving a diameter in the range of 50-62 um. The imaging drum can writeat a resolution in the range of 1200-2400 dpi at speeds of 250-1200 rpm.

While the preferred embodiment sets forth that the focusing beampreferentially has a wavelength of 960 nm, it will be appreciated thatalternative wavelengths may be chosen so long as they are sufficientlydifferent from the predominent wavelength of the writing beam as to bereadily distinguishable therefrom. Moreover, these alternative lightbeam wavelengths may or may not be relatively unabsorbed by the dyelayers, so long as a sufficient amount can be detected at thephotodetector to operate the focus detection system, and the absorptionof the focusing light is insufficient to transfer dye from the donor tothe receiver.

A further alternative to the preferred embodiment may be found in thesurface chosen from which to reflect the focus beam. While thereflective surface of the receiver element is preferred, it is possibleto reflect the focus beam from the surface of the drum member,particularly if the receiver element is transparent, or if the drumsurface is particularly reflective. Still further, other surfaces of thewriting element may be chosen as the surface from which to reflect thefocus beam.

Additional variations in the present invention relate to the placementof the photo detector. For example it may be located outside, butadjacent the writing head so that the reflected portion of the focusingbeam need not pass through the focusing assembly. Further, it ispossible to locate the photodetector behind a transparent surface of thesupport member so that it responds to the direct impingement of thefocusing beam without requiring any reflection thereof.

Accordingly, the present invention provides a method and apparatus forconsistently, quickly and accurately generating an image utilizing sucha thermal imaging process to create high quality, accurate, andconsistent proof images, which method and apparatus is substantiallyautomated to improve the control, quality and productivity of theproofing process while minimizing the attendance and labor necessary.Moreover, the writing apparatus is capable of not only generating thishigh quality image consistently, but is capable of creating amulti-color image which is in registration regardless of how the variousindividual images are supplied to the element comprising the finalimage. Thus, the present invention provides both a method and apparatusin which the various donor material sheets are sequentially superposedwith a single receiver sheet and then removed without disturbing thereceiver sheet on the writing drum or platen, maintaining the receiversheet in one position during the entire writing process to assure thenecessary registration of the multiple superposed images that create thefinal proof.

The invention has been described in detail with particular reference toa presently preferred embodiment, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

I claim:
 1. An imaging drum for a thermal imaging apparatus comprising ahollow imaging drum member mounted for rotation about its axis andarranged to mount a receiver member and a donor member in superposedrelationship thereon, said receiver member having a first width andlength and said donor members having a second width and length greaterthan those of said receiver member, means for providing a vacuum to theinterior of said imaging drum, means for projecting a light beam ontosaid donor member mounted on said drum member to transfer an image ontosaid receiver member by transfer of a dye from said donor member, meansfor rotating said drum member, said imaging drum member being providedwith a first set of vacuum openings through the surface thereof foradhering a receiver sheet thereto, said imaging drum member beingprovided with a second set of vacuum openings disposed about said firstset of vacuum openings for adhering a donor sheet to said drum member insuperposed relation with said receiver sheet adhered thereto, an axiallyextending planar area disposed in the surface of said imaging drummember which is arranged to accept the leading and trailing edges ofsaid donor member, and a receiver-sheet-receiving circumferential recessin the surface of said imaging drum member, said drum diameter and thewidth of the planar area being selected so that the leading and thetrailing edges of said donor sheet overlie the opposite edges of saidplanar area without overlapping each other.
 2. An imaging drum accordingto claim 1 wherein said circumferential recess contains substantiallyall of said first set of vacuum openings in said drum.
 3. An imagingdrum according to claim 1 wherein said circumferential recess extendsfrom one end of said drum to the other but is spaced from each end toprovide a non-recessed area at each end, and wherein the lateral edgesof said donor sheet overlie said non-recessed areas at each end of saiddrum.
 4. An imaging drum according to claim 1 wherein saidcircumferential recess has a depth substantially equal to the thicknessof the receiver sheet to be accepted therein.